1. Technical Field
An embodiment of the present disclosure relates to a voltage booster, particularly for the use in Integrated Circuits (IC).
2. Description of the Related Art
Voltage boosters are used to generate voltages higher, or of opposite polarity than a supply voltage thereof. For example, voltage boosters are integrated in non-volatile semiconductor memory device ICs, which need different voltages depending on the operation that has to be performed on the memory cells; in particular, a program operation and an erase operation of the semiconductor memory device typically require relatively high voltages (higher than the IC supply voltage) or negative voltages (compared to the IC reference voltage). Voltage boosters are in particular exploited to generate on-chip the voltages necessary for the IC operation, when such voltages are not supplied to the IC from the outside.
Typically, the voltage booster includes a charge pump, i.e., a circuit for boosting voltages starting from an input voltage lower, or of different polarity than the required voltage.
The operation of a charge pump is based on the continuous accumulation and transfer of electric charge through cascade-connected circuital stages, including charge-storage elements, particularly capacitors. In this way, a voltage across the capacitors increases moving from a charge pump input terminal, that receives the input voltage, to a charge pump output terminal, at which a boosted voltage is made available. Each stage of the charge pump is selectively coupled to adjacent stages by means of electronic switches, which alternately close and open for accumulating and then for transferring electric charge. The operation of the charge pump is controlled by properly phased periodic timing signals.
The output terminal of the charge pump is coupled to a circuital load, represented by the circuit structures that receive the boosted voltage, and that sink a corresponding current. The impedance of the circuital load may significantly vary; for example, this occurs in a memory device, in which a different number of memory cells can be programmed or erased at a time. Depending on the impedance of the circuital load, the current sunk from the charge pump varies, and accordingly the electric charge transfer rate necessary for sustaining the sunk current varies. In order to keep the boosted voltage close to a target value, the frequency of the charge pump timing signals has to increase or to decrease in accordance with the increase or the decrease of the current sunk (so as to correspondingly increase or decrease the electric charge transfer rate).
The timing signals are typically generated by an oscillator circuit. A voltage regulator receives the boosted voltage at the charge pump output and produces corresponding regulation signals, indicative of the difference between the boosted voltage and a reference voltage. The regulation signals are supplied to the oscillator, to modulate a frequency of the charge pump timing signals.
Usually, a continuous, i.e. analog frequency modulation of the timing signals is provided. In the design of voltage boosters, account must be taken of the particular circuital load of the charge pump (such as the particular memory device) and the technology exploited for the integration process. Accordingly, a voltage booster operating with analog frequency modulation has a relatively complex design that, in addition, does not ensure a stability against process parameters spreads.
Furthermore, a voltage booster with analog frequency modulation has a relatively slow response, so it may not be suitable for applications involving impulsive power consumption, and occupies a relatively large chip area, due to the need of providing compensation capacitors for ensuring stability of the feedback loop. This opposes the continuous quest for speeding up and shrinking ICs, such as memory.